"We need to break the Russian artillery advantage... We need much more modern systems, modern artillery," Zelensky told a NATO summit in Madrid via video link.

He added that financial support was "no less important than aid with weapons".

"Russia still receives billions every day and spends them on war. We have a multibillion-dollar deficit, we don't have oil and gas to cover it," Zelensky said, adding that Ukraine needs around $5 billion a month for its defence.

He also called for sanctions on Russia "that will stop its ability to pay for the war".

Washington and Brussels have slapped Moscow with unprecedented sanctions over its invasion of pro-Western Ukraine that President Vladimir Putin launched on February 24.

The United States and Canada, which are far less reliant on Russia as an energy supplier than Europe, have banned all Russian oil imports.

The European Union, however, has introduced a gradual oil embargo as part of its sanctions on Moscow.